netdev - Re: [PATCH net-next v5] rtnetlink: add new RTM

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Date:	Tue, 19 Apr 2016 19:53:39 -0600
From:	David Ahern <dsa@...ulusnetworks.com>
To:	Johannes Berg <johannes@...solutions.net>,
	David Miller <davem@...emloft.net>
Cc:	eric.dumazet@...il.com, roopa@...ulusnetworks.com,
	netdev@...r.kernel.org, jhs@...atatu.com, tgraf@...g.ch,
	nicolas.dichtel@...nd.com, egrumbach@...il.com
Subject: Re: [PATCH net-next v5] rtnetlink: add new RTM_GETSTATS message to
 dump link stats

On 4/19/16 1:41 PM, Johannes Berg wrote:
> On Tue, 2016-04-19 at 14:23 -0400, David Miller wrote:
>>
>> I like this nlattr flag idea, it's opt-in and any tool can be updated
>> to use the new facility.
>
> Right.
>
>> I'd be willing the backport the nlattr flag bit change to all stable
>> releases as well.
>
> I'm not really convinced that helps much - tools still can't really
> rely on the kernel supporting it.
>
> One thing that might work is that a tool might say it only wants to
> support kernels that have this change (assuming we backport it to
> enough kernels etc.); in that case the tool could add some absolutely
> must-have information (like the IFINDEX or whatever, depends on the
> command) with the new flag, this would get rejected since unpatched
> kernels wouldn't understand the flag and wouldn't find that must-have
> information.
>
> Nevertheless, I think it's most reliable with new netlink commands that
> are known to be available only on kernels understand and treating the
> flag correctly.

The kernel can set a flag in the response that it acknowledges the new 
attribute/flag. I did that for filtering neigh dumps -- 21fdd092acc7.